opI18N stores cached texts in opI18N::$parsed. A new entry of opI18N::$parsed is added by end of opI18N::__() to skip parsing from the next time. But checking available caches in opI18N::__() is wrong.

The above is the main point in this issue. Calling empty() looks like a checking available caches, but this empty() calls to check the undefined local variable $parsed, not $this->parsed. In this code, $parsed is always an empty value. So you get non-cached result from opI18N::__().